Artificial and supplemental lighting in e.g. a greenhouse typically involves use of an illumination system for stimulating plant growth, the illumination system comprising a plurality of high power light sources. Different types of light sources, having different light spectrum and providing different effects on growth stimulation, may be included, such as light sources based on metal halide (MH) or high intensity discharge (HID) which includes high pressure sodium (HPS). Using metal halide based lighting typically promotes shorter, bushy growth; whereas high pressure sodium based lighting in comparison typically tends to produce taller and stretched plants. Thus, different types of light sources may lead to different end growth results of plants, which reflect a challenge in accurate control of the growth of plants with artificial lighting.
Recently, much progress has been made in increasing the brightness of light emitting diodes (LEDs). As a result, LEDs have become sufficiently bright and inexpensive to serve also for artificial lighting in e.g. a greenhouse environment, additionally providing the possibility of emitting light with adjustable light spectrum and intensity offset. By mixing differently colored LEDs and adjusting their individual intensity a desired light spectrum may be obtained. An adjustable lighting system typically comprises a number of primary colors, for example the three primaries red, green and blue. The light spectrum of the generated light is determined by the colors of the LEDs that are used, as well as by the mixing ratios of the intensities of light from LEDs of different colors. Thus, each LED of a LED based illumination system may be individually controlled such that a light spectrum of the overall emitted light may be conveniently adjusted. In addition, LEDs may switched on and off at a high rate and with a wide bandwidth providing the possibility of creating well defined light wave-forms and signals. Furthermore, by using LEDs it is possible to decrease the energy consumption, a requirement that is well in line with the current environmental trend. Additionally, using LED based illumination system minimizes the amount of light source generated heat which is particularly suitable in an environment where temperature control is desirable. An example of an LED based illumination system is disclosed in WO2008118080, comprising a light sensor communicatively coupled to a processor, where the processor implements a control algorithm for modulating and improving plant growth and attributes by adjusting the light spectrum emitted by the illumination system.
Document WO2008118080 shows a promising approach in relation to artificial lighting in a greenhouse environment. However, it would still be desirable to further optimize the artificial lighting systems, to be able to improve the growth process of a plant, specifically improving the diagnosis of plants and accuracy in the determination of growth status for plants arranged in a greenhouse environment. For example, it would be desirable to reduce the importance of the absolute levels of detected light emitted from the plant, e.g. fluorescence signals, for determining a plant growth status. This may, for example, reduce the impact of the distance from the plant to a sensor, or impact of the leaf area on the determination.
Other examples of known techniques include FLIDAR, t-LIDAR, and the JIP test. These techniques suffer from at least some of the above drawbacks. Furthermore, these techniques typically rely on evaluating a transient response of the plant starting from a dark adapted state which makes the evaluation rather complicated for field testing and unsuitable for green house applications. An example of such technique is disclosed in STRASSER et al. ‘Analysis of the chlorophyll a fluorescence transient’, in: Advances in photosynthesis and respiration, 2004, Vol. 19, pages 321-362, ISSN 1572-0233. However, in accordance with the above, it would be desirable to further improve growth status evaluation, and in particular reduce the impact of the environment where the evaluations are performed.